1. Field of the Invention
This invention relates in general to a light deflection element and light deflection device that are capable of changing the propagation direction of light by electrical signals More specifically this invention relates to an image display device using the above light deflection element and light deflection device.
2. Description of Related Art
In the specification, a light deflection element is used for deflecting the optical path by electrical signals supplied externally. Namely, with respect to an incident light, an outgoing light is parallel shifted or is rotated by a certain angle. Alternatively, the light deflection element can alter the optical path by combining the above two methods. In the description, regarding the light deflection due to the parallel shift, its magnitude of shift is known as a shift amount; regarding the light deflection due to the rotation, its rotational amount is known as a rotational angle. The light deflection device contains the above light deflection element(s) to deflect the optical path.
A pixel shift comprises an image display element that includes a plurality of pixels arranged in a two dimensional array and is capable of controlling light based upon at least one image information; a light source for illuminating the image display element; an optical device for observing image patterns displayed on the image display element; and a light deflection element and a light deflection apparatus for deflecting the optical path between the optical element and image display element, wherein the image field is divided into a plurality of sub-fields in the time domain by the light deflection apparatus. By the light deflection apparatus, the image patterns are displayed in a state that the display position is shifted according to the deflection of the optical path for each sub-field. The appearance pixel number of the transmission type liquid crystal panel 46 is doubled to be displayed. Therefore, the light deflection element or light deflection device can be used as the light deflection apparatus.
Conventionally, optical devices consisting of the light deflection element are well known as optoelectronic devices that use a material having a large first order electro-optic effect (the Pockels effect) such as KH2PO4(KDP), NH4H2PO4(ADP), LiNbO3, LiTaO3, GaAs, CdTe etc. or a material having a large second order electro-optic effect such as KTN, SrTiO3, CS2, etc. In addition, audio optical devices that use a material of glass, silica etc. are well known (referring to “optoelectronic device”, edited by Aoki). In general, a long optical path is necessary for obtaining a very large light deflection and applications are limited because the material is expensive.
On the other hand, there are many proposals related to the optical device that consists of the light deflection element using liquid crystal material. Following descriptions provide several examples briefly.
First, in the Japanese Laid-Open 9-18940, a light beam shifter composed of an artificial birefringence plate is proposed in order to reduce the light loss due to optical spatial switch. According to the disclosure, the light beam shifter is constituted by disposing two sheets of wedged-shaped transparent substrates opposite from each other and holding a liquid crystal layer between the transparent substrates. The light beam shifter is connected with the light beam shifters to the rear surface of a matrix type deflection control element. Two sheets of the wedge-shaped transparent substrates are disposed opposite from each other and, therefore, the light beam shifter with which matrix driving is possible and which is connected in multiple stages with the optical beam shifters holding the liquid crystal layers for shifting incident optical beams by a half cell by shifting the shifters by a half cell each is obtained.
Additionally, in the Japanese Laid Open 9-133904, a light deflection switch, that is capable of obtaining a large deflection, a high deflection efficiency and freely setting a deflection distance and a deflection angle, is disclosed. For example, the liquid crystal element has two transparent substrates that are oppositely arranged by a predetermined spacing, and the opposite surfaces of the transparent substrates are perpendicular. A ferroelectric liquid crystal of smectic A phase is sealed between the two transparent substrates. A pair of electrodes is set such that an AC (alternative current) electric field can be applied perpendicular to the two transparent substrates and parallel to the liquid crystal (smectic layer) layer. In addition, a driving device is used for applying the AC electric field to the electrode pair. Namely, by utilizing the electric tilt effect of the ferroelectric liquid crystal of smectic A phase, the refraction angle of the polarized light incident to the liquid crystal layer and the displacement direction can be changed due to the birefringence of the tilt of the liquid crystal molecules.
According to the Japanese Laid-Open 9-18940, because the nematic liquid crystal is used, it is very difficult to have a response time of a sub-millisecond order, thereby it cannot be an application for a high-speed switching function.
In addition, according to the Japanese Laid Open 9-133904, the ferroelectric smectic A phase liquid crystal is used. However, because the smectic A phase liquid crystal doesn't possess a spontaneous polarization, a high-speed operation cannot be operated.
Next, there are many proposals related to the pixel shift element. Following descriptions provide several examples briefly.
For example, in the Japanese Patent 2939826, a projecting display device for enlarging and projecting an image displayed on a display element onto a screen by an optical projection system. The projecting display device comprises a shifting device and a projecting device. The shifting device further comprises at least one optical element capable of rotating the polarization direction of the transmitted light along the optical path from the display element to the screen, and at least one transparent element possessing a birefringence effect. The projecting device can effectively reduce the aperture and project the projection areas of each image on the display element onto the screen.
According to the disclosure of the previous Japanese patent, the pixel shift is performed by a projection image shifting (pixel shifting) device having at least one transparent element (birefringence element) possessing a birefringence effect and at least one optical element (chiral element) capable of rotating the polarization direction. However, because the chiral element and the birefringence element are used together, there are the following problems for example. The light loss is large and the variation of the pixel shift amount due to the wavelength of the light will reduce the resolution. If the optical properties of the chiral element and the birefringence element are mismatched, optical noise, such as ghosting, occurs because light leakage occurs outside the pixel shift position so that the no image will be formed. In particular, the above problem becomes obvious when the birefringence element uses materials having a large first order electro-optical (Pockels) effect, such as KH2PO4 (KDP), NH4H2PO4(ADP), LiNbO3, LiTaO3, GaAs, and CdTe etc.
Additionally, in the Japanese Laid Open 5-313116, a projector is disclosed. A control circuit samples an image to originally be displayed which is stored in an image storage circuit in a pixel selecting circuit in a checked pattern and displays and projects it on a spatial optical modulator in order. Further, a control circuit controls a panel rocking mechanism corresponding to the display and reproduces the image to originally be displayed by composition hourly while shifting a pitch interval between adjacent image elements of a spatial optical modulator by 1/n times (n: integer). Consequently, the image can be displayed with resolution that is the integral multiple of the image elements of the spatial optical modulator and the projector can be constituted at low cost by using the spatial optical modulator having scattered image elements and a simple optical system.
Also, the Japanese Laid Open 5-313116 discloses a pixel shifting method for shaking the image display element by a distance smaller than the image pitch at a high speed. Regarding this method, because the optical system is fixed, the aberration seldom occurs. However, because the image display element has to be accurately and quickly moved in parallel, the accuracy and the durability required for the movable parts cause problems such as vibration or noise.
In the Japanese L aid Open 6-324320, the apparatus resolution of an image displayed on an image display system is increased without increasing the number of actual pixels which are arranged in horizontal rows and vertical columns and selectively energizable to display an image composed of a plurality of pixel patterns in alternate fields. An optical element is positioned between the image display system and a viewer or screen for shifting an optical path there between to optically shift a pixel pattern. The optical element is operated to shift the optical path, and the pixel pattern to be optically shifted is displayed on the image display system in every field according to the shifting of the optical path by the optical element. In every field of the image information, the portions where the indexes of refraction are different appear alternatively in the optical path between the image display system and a viewer or screen, thereby the optical path is altered.
Also, the Japanese Laid Open 6-324320 describes an apparatus for shifting the optical path that can be a mechanism of a combination of a opto-electronic element and a birefringence material, a lens shifting mechanism, a vary-angle prism, a rotational mirror or a rotational glass etc. In addition to the combination of the opto-electronic element and the birefringence material, the gazette also discloses that the optical element (for example, a lens, a reflecting plate or a birefringent plate) is displaced (parallel moved or tilted) by such as a voice coil or piezoelectric plate to switch the optical path. However, in this method, in order to drive the optical element, the structure becomes complicated and the cost increases.
In addition, according to the Japanese Laid Open 10-133135, a light beam deflection device is disclosed where no rotational mechanical parts are required, thereby a smaller, highly-accurate and high-disassemble device can be made and external vibration is hard to affect the light beam deflection device. According to the disclosure, the light beam deflection device comprises a light-transmissive piezoelectric element arranged in the optical path, transparent electrodes formed on the surface of the light-transmissive piezoelectric element and a voltage applying device for applying a voltage on the light-transmissive piezoelectric element through the transparent electrodes such that the optical axis of the light beam is deflected by changing the length of the optical path between the incidence surface A and projection surface B of the piezoelectric element.
In the previous disclosure, the transparent electrodes sandwich the light-transmissive piezoelectric element and a voltage is applied to the transparent electrodes such that the thickness of the light-transmissive piezoelectric element is changed to shift the optical path. However, a larger light-transmissive piezoelectric element is required and therefore the device cost increases, which has the same problems mentioned in the Japanese Laid Open 6-324320.
In summary, the conventional pixel shift element has the following disadvantages:    (1) Due to its complicated structure, problems, such as the high cost, the enlarged device, the light loss or the optical noise of ghosting etc., occur.    (2) Due to the movable parts, there are problems such as the position accuracy, durability, vibration and noise etc.    (3) Due to the nematic liquid crystal, the response time is slow.
Regarding the response time, the response time for the required light deflection of the pixel shift in the image display device can be estimated as follows. An image field is divided into n in the time domain (time tField). If the optical path between the image display device and the optical element is deflected in every n sub-field to fix the shifting position of the pixel shift at n points, the time for one sub-field can be expressed by tSF=tField/n. The light deflection is performed in the interval of time tSF. When the time is tshift, there is no display in the interval of time tshift. Therefore, the utility efficiency of the light becomes lower in this time interval.
The utility efficiency of the light E can be expressed by E=(tSF−tshift)/tSF. Assuming the pixel shift position n is n=4 and the image field tField is 16.7 ms, in order to maintain the utility efficiency of the light E above 90%, tshift can be calculated from 0.9 <(16.7/4−tshift)/(16.7/4). As a result, tshift satisfies tshift<0.42(ms). Namely, the light deflection has to be 42 ms. However, because the response time for a nematic liquid crystal is several milliseconds, the conventional technology cannot apply to the optical device for high-speed pixel shift.
In the Japanese Laid Open 6-18940, because the nematic liquid crystal material is used, it is very difficult to have a response time of a sub-millisecond order, thereby it cannot be an application for a pixel shift device. However, the response time of a ferroelectric chiral smectic C phase liquid crystal can be set under 0.42 ms.
In addition, according to the Japanese Laid Open 9-133904, the ferroelectric smectic A phase liquid crystal is used. However, because the smectic A phase liquid crystal doesn't possess a spontaneous polarization, the high-speed operation that a chiral smectic C phase liquid crystal can provide can hardly be expected.